Introduce filtergraph customization

[hheimbuerger]

This is a continuation of #59. Sorry, I couldn't be bothered to separate the branches. But I still wanted to create a separate PR, because this is might be a very controversial additional.

Allows optionally defining filtergraphs for VOD video and audio profiles. These work on top of the existing automatic rescaling filter.

Example:

vod:
  video-profiles:
    1080p:
      width: 1920
      height: 1080
      bitrate: 5000
      filtergraph:
        - "[vin]format=pix_fmts=yuv420p[vout]"   # change pixel format to yuv420p

vod:
  audio-profile:
    filtergraph:
      - "[ain]asetrate=48000*1.5,aresample=48000[aout]"   # Pitch the audio up by ~50 % (everyone's sounds like that famous mouse!)